Evolution of C^IV^ absorbers. I. Absorber properties

By searching 369 high-resolution, high signal-to-noise ratio spectra of quasars at 1.1<=z_em_<=5.3 from Keck/HIRES and VLT/UVES, we find 1268 C^IV^ absorbers with W_r_>~0.05{AA} (our ~50% completeness limit) at redshifts 1.0<=z<=4.75. A Schechter function describes the observed equivalent width distribution with a transition from power-law to exponential decline at W_r_>~0.5{AA} .

The power-law slope {alpha} rises by ~7%, and the transition equivalent width W* falls by ~20% from <z>=1.7 to 3.6. We find that the comoving redshift path density, dN/dX, of W_r_>=0.05{AA} absorbers rises by ~1.8 times from z~4.0 to 1.3, while the W_r_>=0.6{AA} dN/dX rises by a factor of ~8.5. We quantify the observed evolution by a model in which dN/dX decreases linearly with increasing redshift.